Secular evolution of the subcontinental lithospheric mantle beneath Indian cratons: Insights from geochemistry and geochronology of the Precambrian mafic dykes

The Indian Shield is comprised of the Dharwar, Bastar, Singhbhum, Bundelkhand, and Aravalli Cratons, intruded by distinct mafic dyke swarms of different generations (ca. 2.8-0.8 Ga). Most of these dykes are tholeiitic basalt to basaltic-andesite, including boninite. Some other subordinate dyke rocks are of komatiitic, picritic, and andesitic compositions. The vast areal extent of these dykes indicates that they are remnants of Precambrian Large Igneous Provinces (LIPs). The present study reviews the existing geochronological and geochemical data of various Precambrian mafic dyke swarms intruding in all Indian cratons, to track temporal changes in composition of the underlying subcontinental lithospheric mantle (SCLM). Most of these dykes have crust-like abundances of incompatible trace-element. Even the primitive dykes (Mg# = 82-64) exhibit crust-like incompatible element patterns. However, some also have depleted mantle-like abundances of incompatible elements. Most of the dykes also have high concentrations of compatible trace elements (e.g., Cr and Ni). The age-corrected radiogenic Nd isotope compositions (epsilon Nd(i)) of these dykes vary between the upper continental crust and depleted mantle epsilon Nd growth arrays. The elemental composition and epsilon Nd(i) of these dykes suggest their derivation from a heterogeneous SCLM comprising enriched (metasomatized) and depleted mantle components. The negative as well as positive epsilon Nd(i) values exhibited by the ca. 2.8 Ga (oldest) dykes suggest the presence of enriched mantle materials distributed within a depleted-SCLM already by ca. 2.8 Ga. The enrichment must have occurred before ca. 2.8 Ga due to subduction-released fluids. Later, the SCLM beneath Indian cratons evolved with a composition consisting of an enriched/metasomatized mantle component and a depleted mantle component.